We are doing a project right now where the old warehouse was found to lack a retarder under the slab. RH came in at 99%. It is being VAP'd this week. What is interesting is when I discussed with my VAP rep, he kept talking about moisture vapor pressure. Can anyone point me to anything linking RH & Vapor? I guess where I am a little confused is how does RH predict Vapor / Osmotic Action?

It is one thing to use a water soluble adhesive that will dissolve under PH & Moisture over time if in contact with concrete water (for lack of a better word) but it is another thing to have that wicking action acting as a force from bellow. The two can result in more blisters (a light version of what you see with ASR). Anyone have a perspective on this?

You have three different issues you are bringing up:
Vapor, RH, and Osmosis.

The RH of a slab can lead to increased vapor drive, as vapor drive is a function of differential vapor pressures. So a wet slab will release more vapor to dry air, and a dry slab will release less, or I should say release vapor at a slower rate.

Osmotic blisters are a completely different beast caused by a difference in salt concentrations on either side of a semi-permeable membrane. The pressures achievable by osmosis are in the hundreds of psi, where vapor pressure will not exceed 1 psi. When you consider even a weak glue will hold at 20 to 30 psi, vapor will never cause a blister.

(10-13-2012, 08:35 PM)CC Solutions Wrote: The RH of a slab can lead to increased vapor drive, as vapor drive is a function of differential vapor pressures. So a wet slab will release more vapor to dry air, and a dry slab will release less, or I should say release vapor at a slower rate.

I see so high RH = higher vapor but we don't have an accurate test for vapor so our best guess is RH as a predictor. Let's leave Osmosis out of the conversation for now. How do I measure vapor if the science behind the ASTM is no good?

I can see this question happening to me and want to have my answer ready for some day...

CaCl is a flawed test but it can be used to show relationships. I wouldn't bet the farm on the readings, but under similar conditions with similar installations you will get similar results.

And to clarify, high RH means higher water content not necessarily higher vapor transmission.

Typically a higher RH slab will have the tendency to emit more vapor because it has a lot more water to emit to reach environmental equilibrium.

I hate all these disclaimers I have to write because guys like Bob or Ed will jump in here and find the one slab that is 90%RH and the surrounding environment is 98%RH and the slab wouldn't be emitting any vapor... it would be absorbing vapor!!!

Which then brings us to the problem with CaCl: What kind of reading will we get on a 90% slab in a 98%RH environment when we place a plastic dome separating the slab from the environment and then place a desiccant in the dome creating an artificially dry environment?? The slab may not be emitting any vapor except under the dome!